I don't see a null pointer message but the b44 driver is still broken in 5.4.0-33-generic on my Dell Inspiron 6400:

[ 3148.539194] ssb: Found chip with id 0x4401, rev 0x02 and package 0x00
[ 3148.599317] b44 0000:03:00.0: Sonics Silicon Backplane found on PCI device 0000:03:00.0
[ 3148.599414] b44: Broadcom 44xx/47xx 10/100 PCI ethernet driver version 2.0
[ 3148.627839] b44 ssb1:0 eth0: Broadcom 44xx/47xx 10/100 PCI ethernet driver 00:15:c5:b3:8a:ba
[ 3148.684246] swiotlb_tbl_map_single: 14 callbacks suppressed
[ 3148.684251] b44 0000:03:00.0: swiotlb buffer is full (sz: 1566 bytes), total 0 (slots), used 0 (slots)
[ 3148.684261] ------------[ cut here ]------------
[ 3148.684267] WARNING: CPU: 0 PID: 36638 at kernel/dma/direct.c:35 report_addr+0x37/0x60
[ 3148.684268] Modules linked in: b44 mii iptable_filter bpfilter ccm b43 dell_laptop dell_smm_hwmon snd_hda_codec_hdmi cordic kvm snd_hda_codec_idt snd_hda_codec_generic bcma ledtrig_audio radeon dell_wmi snd_hda_intel snd_intel_dspcfg dell_smbios dcdbas input_leds joydev mac80211 snd_hda_codec r852 sparse_keymap wmi_bmof serio_raw dell_wmi_descriptor ttm sm_common snd_hda_core nand snd_hwdep drm_kms_helper nand_ecc snd_pcm cfg80211 bch i2c_algo_bit snd_timer fb_sys_fops r592 nandcore mtd memstick syscopyarea sysfillrect snd libarc4 sysimgblt soundcore mac_hid sch_fq_codel coretemp parport_pc ppdev lp parport drm ip_tables x_tables autofs4 ssb_hcd gpio_ich pata_acpi i2c_i801 psmouse lpc_ich firewire_ohci sdhci_pci cqhci firewire_core sdhci crc_itu_t ssb wmi video
[ 3148.684314] CPU: 0 PID: 36638 Comm: NetworkManager Not tainted 5.4.0-33-generic #37-Ubuntu
[ 3148.684315] Hardware name: Dell Inc. MM061                           /0XD720, BIOS A17 06/13/2007
[ 3148.684318] RIP: 0010:report_addr+0x37/0x60
[ 3148.684320] Code: 48 8b 87 28 02 00 00 48 89 75 f8 48 85 c0 74 2a 4c 8b 00 b8 fe ff ff ff 49 39 c0 76 11 80 3d db c3 71 01 00 0f 84 7d 07 00 00 <0f> 0b c9 c3 48 83 bf 38 02 00 00 00 74 f2 eb e3 80 3d bf c3 71 01
[ 3148.684321] RSP: 0018:ffff9ae68055b450 EFLAGS: 00010246
[ 3148.684323] RAX: 00000000fffffffe RBX: 0000000000000000 RCX: 0000000000000000
[ 3148.684324] RDX: 000000000000061e RSI: 000000006b440a80 RDI: ffff8cb34b4db0b0
[ 3148.684325] RBP: ffff9ae68055b458 R08: 000000003fffffff R09: 0000000000000004
[ 3148.684327] R10: 0000000000000000 R11: 0000000000000001 R12: ffff8cb2f8b0c000
[ 3148.684328] R13: 000000000000061e R14: ffff8cb2f45cef00 R15: ffff8cb2f96ac8c0
[ 3148.684330] FS:  00007f02e05ca240(0000) GS:ffff8cb34c600000(0000) knlGS:0000000000000000
[ 3148.684331] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3148.684333] CR2: 00007f5935e43f28 CR3: 00000000c6128000 CR4: 00000000000006f0
[ 3148.684334] Call Trace:
[ 3148.684340]  dma_direct_map_page+0xdc/0xf0
[ 3148.684347]  b44_alloc_rx_skb+0x1bb/0x4a0 [b44]
[ 3148.684348]  ? dma_direct_alloc_pages+0x93/0x130
[ 3148.684351]  b44_init_rings+0xb8/0x190 [b44]
[ 3148.684354]  b44_open+0xfc/0x470 [b44]
[ 3148.684357]  __dev_open+0xd4/0x170
[ 3148.684359]  __dev_change_flags+0x191/0x1f0
[ 3148.684361]  dev_change_flags+0x26/0x60
[ 3148.684364]  do_setlink+0x82e/0x980
[ 3148.684368]  ? cpumask_next+0x1b/0x20
[ 3148.684371]  ? __snmp6_fill_stats64.isra.0+0xe7/0x110
[ 3148.684375]  ? __nla_validate_parse+0x42/0x140
[ 3148.684377]  __rtnl_newlink+0x580/0x970
[ 3148.684380]  ? __nla_put+0x20/0x30
[ 3148.684383]  ? __kmalloc_node_track_caller+0x205/0x320
[ 3148.684385]  ? pskb_expand_head+0x75/0x2d0
[ 3148.684387]  ? __kmalloc_reserve.isra.0+0x31/0x90
[ 3148.684391]  ? security_sock_rcv_skb+0x2f/0x50
[ 3148.684393]  ? sk_filter_trim_cap+0x48/0x220
[ 3148.684395]  ? skb_queue_tail+0x43/0x50
[ 3148.684397]  ? __netlink_sendskb+0x42/0x50
[ 3148.684401]  ? security_capable+0x3d/0x60
[ 3148.684404]  ? _cond_resched+0x19/0x30
[ 3148.684406]  ? kmem_cache_alloc_trace+0x16c/0x240
[ 3148.684408]  rtnl_newlink+0x49/0x70
[ 3148.684410]  rtnetlink_rcv_msg+0x2c0/0x380
[ 3148.684413]  ? vsnprintf+0x35d/0x4e0
[ 3148.684414]  ? rtnl_calcit.isra.0+0x100/0x100
[ 3148.684416]  netlink_rcv_skb+0x50/0x120
[ 3148.684418]  rtnetlink_rcv+0x15/0x20
[ 3148.684420]  netlink_unicast+0x187/0x220
[ 3148.684422]  netlink_sendmsg+0x222/0x3e0
[ 3148.684425]  sock_sendmsg+0x65/0x70
[ 3148.684426]  ____sys_sendmsg+0x212/0x280
[ 3148.684429]  ___sys_sendmsg+0x88/0xd0
[ 3148.684431]  ? rtnl_unlock+0xe/0x10
[ 3148.684433]  ? addrconf_sysctl_forward+0x111/0x280
[ 3148.684436]  ? get_max_files+0x20/0x20
[ 3148.684439]  ? __fget_light+0x57/0x70
[ 3148.684441]  __sys_sendmsg+0x5c/0xa0
[ 3148.684444]  __x64_sys_sendmsg+0x1f/0x30
[ 3148.684447]  do_syscall_64+0x57/0x190
[ 3148.684450]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 3148.684452] RIP: 0033:0x7f02e1a3b12d
[ 3148.684454] Code: 28 89 54 24 1c 48 89 74 24 10 89 7c 24 08 e8 ca ee ff ff 8b 54 24 1c 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 2f 44 89 c7 48 89 44 24 08 e8 fe ee ff ff 48
[ 3148.684455] RSP: 002b:00007fffaac61a40 EFLAGS: 00000293 ORIG_RAX: 000000000000002e
[ 3148.684457] RAX: ffffffffffffffda RBX: 000056483eb4a880 RCX: 00007f02e1a3b12d
[ 3148.684458] RDX: 0000000000000000 RSI: 00007fffaac61a90 RDI: 000000000000000c
[ 3148.684459] RBP: 00007fffaac61a90 R08: 0000000000000000 R09: 0000000000000000
[ 3148.684460] R10: 000056483eb1c010 R11: 0000000000000293 R12: 000056483eb4a880
[ 3148.684462] R13: 00007fffaac61c48 R14: 00007fffaac61c3c R15: 0000000000000000
[ 3148.684464] ---[ end trace 27a392bd04b8574c ]---
[ 3148.684481] b44 0000:03:00.0: swiotlb buffer is full (sz: 1566 bytes), total 0 (slots), used 0 (slots)
[ 3148.684497] b44 0000:03:00.0: swiotlb buffer is full (sz: 1566 bytes), total 0 (slots), used 0 (slots)
[ 3148.684502] b44 0000:03:00.0: swiotlb buffer is full (sz: 1566 bytes), total 0 (slots), used 0 (slots)
[ 3148.684507] b44 0000:03:00.0: swiotlb buffer is full (sz: 1566 bytes), total 0 (slots), used 0 (slots)
[ 3148.684512] b44 0000:03:00.0: swiotlb buffer is full (sz: 1566 bytes), total 0 (slots), used 0 (slots)
[ 3148.684517] b44 0000:03:00.0: swiotlb buffer is full (sz: 1566 bytes), total 0 (slots), used 0 (slots)
[ 3148.684522] b44 0000:03:00.0: swiotlb buffer is full (sz: 1566 bytes), total 0 (slots), used 0 (slots)
[ 3148.684527] b44 0000:03:00.0: swiotlb buffer is full (sz: 1566 bytes), total 0 (slots), used 0 (slots)
[ 3148.684532] b44 0000:03:00.0: swiotlb buffer is full (sz: 1566 bytes), total 0 (slots), used 0 (slots)
bgjenero@pacifica:~$ uname -a
Linux pacifica 5.4.0-33-generic #37-Ubuntu SMP Thu May 21 12:53:59 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux